(1) Field of the Invention
The present invention relates to organic synthetic thermoplastic building enclosure members and more particularly to skylights.
(2) Description of the Prior Art
Thermoplastic compounds have recently found wide-spread utilization as building materials. These compounds have inherent characteristics which make them translucent to light with good weatherability. However, when such compounds are utilized as enclosures for buildings, especially for a roofing or panel application, the structural characteristics of the compounds for enclosing large areas are such that additional framing members need be employed to prevent setting or collapse when subjected to unusual structural loads thereon.
One particular application where thermoplastic material is utilized in skylight construction and the like is in roofs of buildings. The use of thermoplastic material for skylights are disclosed in U.S. Pat. Nos. 2,610,593 and 2,790,400 to Wasserman and 3,434,250 to Kiekhaefer. These patents generally describe dome and dual dome shapes which have been limited to small and medium skylight openings since plastic domes cannot be made and handled efficiently in the larger sizes. Furthermore, from an aesthetic standpoint, the dome-shaped skylights generally break symmetrical lines. The current design trend is to a flat shape or low profile skylight which blends into the basic building design and maintains architectural integrity. Thermoplastics are desireable as safety glazing. Flat thermoplastics used overhead or vertically as transparent glazing or opaque decorative panels do not remain flat. As a consequence of temperature differentials from inside to outside, material fabricated originally as flat begins to bow inward due to the expansion of the inner surface with higher temperature and greater humidity absorption on the inside. Thermoplastics originally flat and after deflection due to temperature or humidity bowing and under live loads such as wind and snow develop into a structurally sound but aesthetically unattractive inwardly bowed concave form. Once bowed due to gradients, conventional flat glazing remains bowed in the same inward direction even though gradients may change with seasons. Stresses under live loads in this concave form are known as diaphragm stresses. These are generally not large and the resulting inwardly bowed concave form is able to support relatively large loads, much larger than conventional upward formed domes of equal thickness. This beneficial load bearing characteristic is also part of this invention since under large loads it develops into the inwardly bowed concave form. However, the new invention has neither the undesireable characteristic of temperature and humidity bowing nor the 5% to 10% rise required in conventional doming processes to produce a minimally useful rigid shape. Moreover, an important characteristic of the panels of this invention is that when the load is removed, the panels return smoothly without "oil-canning" or sharp rebounding to their original modified flat form, having a slight upward bow. By contrast, conventional flat panels retain a slight inward bow due to temperature and humidity gradients. This can result in an undesireable and unattractive sagging appearance, with a further resultant tendency to collect water and dirt in the inwardly concave bow.
Additionally, conventional domes of greater thickness fracture upon buckling under load, since the curvature reversing process produces destructively high tensile stresses at the perimeter of the dome. The present commercially available low profile (flat) products are manufactured using a free blown process which applies air pressure to the bottom side of a formed thermoplastic sheet in a hot plastic state, or by molding the hot sheet by molding with a shaped bottom plug, to effect a deflection of 5% to 10% of the short dimension in the final product. Forming to only 5% to 10% is a compromise on engineering to obtain better aesthetics. The dome rise which produces optimum strength is 25% of the shorter base dimension. This is documented in publications of the Rohm & Haas Company, 1971.
By contrast, a thermoplastic skylight, or the like, produced according to this invention overcomes these difficulties of the prior art low profile constructions by providing not only an improved member which resists loads and bowing due to temperature and humidity gradients but provides extra rigidity with the lowest possible silhouette (curvature ranging from 0.5% to 2.0% of the short dimension) but, also, has a spring-back property not found in current structures. That is, the building enclosure member constructed in accordance with the present invention, withstands more substantial loads than currently available enclosure members and if subjected to unusual deforming loads will spring-back to its original shape and condition.